All terrain tires

Hi all, I'm new here. I have an 09 silverado with the 5.3L v8 in it. I'm wanting to put all terrain tires on my truck to give it a more aggressive look as well as be an aid when I'm in the woods. So, my question is this, how badly will my gas mileage be affected by going from the factory 275/55/20 street tires to the same size but using the nitto terra grapplers instead. If it's not a terrible drop I had considered going with a more aggressive look and going with 275/70/18, but I know that that size is 33" tall compared to the factory 31.9. Any idea on how badly the gas mileage will be affected with this setup also?

When you upsize tires significantly from OEM primary or alternate sizes expect to lose 1-3 MPG. 33 or larger inch tires are big and heavy ... and you have to get those suckers rolling and keep them rolling. Unless you regear to match the new size (to a close approximation of what you had, RPM-wise, at certain speeds on your stock tires and gears) you're going to lose MPG because you downshift to ascend mild inclines where you didn't have to, before. You're also taller so you've got more air resistance.

You can get an electronic tuner and use a stock tune on it to recover some of this. You can recover a bit more of it with a custom tune via the tuner. In addition to a tuner it'll take intake/exhaust work and the regear I mentioned to recover the lion's share of what you lose from lifts and upsized tires.

Surreal

P.S. You'll also want a tuner to correct the speedo error you'll induce by going with larger tires.

My stock tires are 245/70R17 and I have on there right now 265/70R17 Wrangler AT/S. I like them and they look good. My speedometer is off a little bit but knowing that, it isn’t too bad. My MPG is about 14-15. If that is what you like and it cost you an extra gallon a week. Is it worth it to you? To me yes...

I went from a 265/65 18 (31.6" dia) stock to a 270/70 18 (33.16" dia). I noticed an immediate drop of about 2 mpg in fuel economy, and my speedo is off by over 3 mph. I called my dealership but they cant reprogram it for anything above stock size tires, so I'm kinda stuck. There may be a few more factors that are affecting my mpg; for instance, I went with a heavy 10-ply tire (mistake) and I only have a 4.8L which I will never have again because I feel it is under powered. But in the long run, I don't really drive many miles a week so it doesn't hit my billfold too much unless I travel on the interstate or tow my trailer around town. If I were you, I would probably base my decision on how much you currently drive and how much a little less mpg would effect you. Good luck on your decision!